Interpretive Summary: Dietary protein supplies lactating dairy cows with amino acids, the building blocks required by the cow to make the proteins in milk and body tissues. About half of the amino acids in protein are essential. This means that they cannot be made by the body, but must be absorbed intact from the intestine to be used to form proteins. Feeding too much protein to dairy cows results in excessive nitrogen excretion, especially in the urine, the form of excretory nitrogen that is the most polluting. Supplementation with the limiting essential amino acid is commonly used in non-ruminant nutrition to allow the feeding of lower protein diets that result in reduced urinary nitrogen excretion. This strategy can now be used in dairy cattle because methionine, an essential amino acid that is often limiting on typical rations fed in North America, is commercially available in rumen-protected form (that is, coated with materials that protect the methionine from breakdown by the microbes living in the rumen, and allowing the amino acid to be absorbed at the intestine when fed in the diet). Two feeding studies were conducted to test whether supplementation with rumen-protected methionine would allow feeding of less protein, thereby reducing urinary nitrogen excretion, without losing milk production. In the first trial, 28 Holstein dairy cows, were fed 4 different diets: protein was reduced from 18.6 to 17.3, 16.1 and 14.8% of dietary dry matter; supplementation of rumen-protected methionine was increased from 0 to 5, 10 and 15 g/day with each step down in dietary protein. Yield of milk, fat-corrected milk, and milk fat were actually greater at 17.3% and 16.1% dietary protein when rumen-protected methionine was fed, than on either higher or lower protein. Although nitrogen efficiency was greatest, and urinary nitrogen excretion lowest, on 14.8% protein, cows were losing protein from their bodies when fed that diet (but not on the other 3 diets). The best compromise among milk production, reduced urinary nitrogen excretion, and cow health occurred on the 16.1% protein diet supplemented with rumen-protected methionine. In the second trial, cows were fed diets containing 16.1 or 17.3% protein, with or without rumen-protected methionine. On average, cows ate about 5 pounds less feed dry matter than in the first experiment, and all cows were losing protein from their bodies. There was no effect of supplementing with the rumen-protected methionine supplementation but feeding the extra protein had a small positive effect on yield of milk and milk protein, and tended to increase feed intake. Mobilization of body protein when feed intake is too low to maintain milk yield appears to have prevented a positive effect of supplementation with rumen-protected methionine. This research indicates that wastage of dietary protein on U.S. dairy farms can be reduced by proper supplementation of rumen-protected methionine. However, this strategy proved ineffective when feed intake was depressed and cows were mobilizing protein from their bodies. These results mean that dairy farmers could use supplementation of rumen-protected methionine to reduce nitrogen pollution of the environment by minimizing protein in the diets they feed their milking cows.